Poor input power factor and high input current total harmonic distortion (THD) generated by phase controlled and uncontrolled diode bridge rectifiers are well known problems in the power converter/rectifier industry. Low power factor and high THD commonly leads to input AC voltage distortions, AC distribution system losses, neutral harmonic currents, excitation of system resonances, and over-rated back-up alternator KVA ratings for telecommunication applications. To combat these problems, designers have attempted to develop improved three-phase rectifiers or converters which draw nearly sinusoidal line currents with low harmonic content and with high displacement power factor.
Boost converters are commonly used in power factor correction AC/DC rectifier applications as line conditioners. Typically, continuous conduction mode (CCM) boost converters are the topology of choice for providing a highly regulated output voltage from substantially lower DC voltages derived from sinusoidal input voltages. The boost stage processes the AC input and develops a DC output voltage, typically 400 V or 800 V. The use of split boost converter topology in high input voltage conditions is particularly attractive because of several advantages, such as reduced boost inductor sizes, lower voltage rating switches and capacitors and higher efficiency.
A split boost topology provides two equal but unparallelable voltages via two output capacitors. Efficiency is achieved by limiting the lowest input voltage to a value of the voltage level of each of the two output voltages. The use of a split boost topology permits the use of power switching semiconductor devices rated at half the breakdown voltage required by a conventional power factor corrected boost stage, while still providing high power conversion efficiency. For example, a DC/DC bridge converter operating with 480 Vac line input and having outputs with an intermediate DC bus voltage of only 400V may employ readily available 600V semiconductor switching devices and 450V aluminum electrolytic energy storage capacitors, without compromising efficiency. The split boost converter optimizes the use of power switches without compromising the advantages found in the three-level boost converter, such as a reduced boost inductor size.
Although the split boost converter provides many advantages, it does not completely solve the poor input power factor and high input total harmonic distortion (THD) problems implicit in three-phase high power rectification applications. For example, the input current drawn by the three-phase split-boost converter exhibits a non-sinusoidal wave-shape of a discontinuous 120.degree. conduction type. An input current waveform of a three-phase, split boost converter while powering a heavy load is typically clipped and appears as a square wave, rather than a true sinusoid. Such low power factor and high THD commonly leads to the problems as addressed above. In an effort to correct these problems, circuit designers have attempted to develop three-phase rectifiers or converters which draw nearly sinusoidal line currents with low harmonic content and high displacement power factor.
Recently, the power supply industry has become very cost sensitive, with low production cost being the key to success. Additionally, three-phase system THD requirements vary, with the international market THD requirement often being in the vicinity of 20 percent, as opposed to the domestic market which can accommodate THD of up to approximately 40 percent. Also, some corrective circuits may require ultra-fast recovery diode bridge rectifiers and a current-limiting turn-on circuit such as a snubber.
Accordingly, what is needed in the art is a three-phase, split boost converter topology that is cost effective, but exhibits a higher power factor while reducing input current THD.